Etching bath for titanium and its alloys and process of etching



United States Patent ETCHING BATH FOR TITANIUM AND ITS ALLOYS AND PROCESS OF ETCHING Walter L. Aclrer, Wetherslield, and Lloyd H. Curkin,

Manchester, Conn., assignors to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware No Drawing. Filed Nov. 20, 1956, Ser. No. 623,288

5 Claims. (Cl. 41-42) The present invention relates to a novel and improved mordant composition especially useful in connection with titanium and its alloys and a process of etching or chemical-milling such metals.

Objects and advantages of the invention will be set forth in part hereinafter and in part will be obvious herefrom, or may be learned by practice with the invention, the same being realized and attained by means of the compositions, processes and steps pointed out in the appended claims.

The invention consists in the novel compositions, processes, steps and improvements herein shown and described.

The present invention has for its object the provision of a novel and improved process for the etching or chemical-milling of titanium metal and titanium alloys. A further object is the provision of a novel and improved mordant composition which acts rapidly on the titanium metal or alloys and which does not cause any substantial hydrogen embrittlement of the metal or alloy. Still another object is the provision of a novel and improved mordant and process for use with titanium metal and alloys which acts rapidly on the metal, leaves the etched metal with an excellent surface finish, avoids the necessity of subsequent vacuum annealing or heat-treatmerit of the alloy or metal to rid it of adsorbed hydrogen, and which may also be used to prepare the metal or alloy for subsequent machining or welding.

In accordance with the composition of the present invention, the mordant comprises an aqueous solution of hydrofluoric acid, nitric acid and a surface active, wetting agent which is relatively stable in the acid solution. This mordant may be used with suitable resists, such as lacquer or bees-wax, for the chemical-milling or etching of a wide variety of titanium alloys as well as of titanium metal, the etching or chemical-milling being controlled by means of a suitable resist or stopping-out material. The same mordant is also useful in removing the surface layer or skin, probably an oxide layer and usually about 0.0015" to 0.002" thick although sometimes as thick as 0.020", which is ordinarily present on the surface of metal objects formed of titanium metal or titanium alloy, and which is to be removed before performing machining or welding operations on the objects.

. Referring now in detail to the composition of matter which forms a part of the present invention, the mordant or etching bath comprises an a ueous .solution of h mggfluog'c acid, nitric acid and a wetting agent, preferably a sulfated fatty alcohol or a sflrum aIEyl sulfate,

which is relatively stable in the mordant and is not rapidly decomposed by the nitric and hydrofluoric acid present in the mordant bath.

The preferred mordant or etching bath is prepared in accordance with the following formula:

Percent by weight Water, enough to make 100.0

"ice

This is conveniently prepared by diluting 8 parts by volume of 60% commercial hydrofluoric acid and 12 parts by volume of commercial, concentrated (69%) nitric acid with approximately 80 parts by volume of water, and adding to it about 0.05 ounce avdp. per gallon of Orvus W.A. Paste (a sulfated fatty alcohol), after which the mordant is ready to use.

In place of using Orvus W.A. Paste, "Petrowet W.N. (a sodium alkyl sulfate) may be substituted with substantially equal effect, neither of these wetting agents being rapidly decomposed by the acids in the mordant or etching bath.

The concentrations of the hydrofluoric and nitric acids may be varied within relatively narrow limits for the purposes of the present invention, and considerable latitude is permitted in the concentration of the wetting agent. The following tabulation shows the good operating ranges for the various components of the mordant:

Hydrofluoric acid From 4.2% to 6.8%

' by weight. Nitric acid From 9.2% to 12.5% by weight. Wetting agent From 0.0015% to 0.08% by weight. Water, enough to make 100.0%.

Less desirably, and in case a slower acting etching bath is suitable for use, the hydrofluoric acid concentration may be reduced to as low as 2.4%, but below 2.4% by weight the etching occurs on the titanium metal or alloys at too slow a rate to be practical. Also, the nitric acid concentration may less desirably be reduced to as low as about 4.6 by weight. Reducing the concentration of the wetting agent much below 0.0015% results in a substantial increase in hydrogen embrittlement, due to the hydrogen which is adsorbed by the titanium metal or alloy on the surface of the metal as it is etched or chemical-milled. Increasing the wetting agent concentration above about 0.08% causes excessive foaming of the etching bath, and is objectionable from that point of view. Likewise, increasing the concentration of the hy-- drofluroic acid above 7.4% in the mordant causes excessively rapid etching or chemical-milling and a resultant poor surface finish on the titanium metal or alloy, and is undesirable for that reason. Increasing the concentration of the nitric acid above 21.2% causes the reaction to be too slow to be practical with 6.8% hydrofluoric acid.

The titanium metal or alloy to be etched or chemicalmilled may be in the form of sheet, finished parts, forging or castings, and these parts are provided with the surface coating of impervious material in those areas which -are not to be etched, after which the alloy or metal is Rem-Cru A 110 AT, an a alloy comprising 5% aluminum, 2.5% tin, 92.5% titanium.

AMS 4900, an a alloy comprising more than 99% titanium. 1

AMS 4901, an e alloy comprising more than 99% titanium.

An up alloy comprising 6% aluminum, 4% vanadium and titanium.

unmilled, stress relieved aesnooa 3 In actual tests of the process of the present invention, notched sheet stock specimens of a titanium alloy comprising:

Carb .20% max. Aluminum 4 t 6%. Ti 2 to 3%. Oxygen 25% max. Nitrogen 10% max. Hydrogen---- 017.5% max. Remainder Titanium.

0.075" thick were chemically milled and were then subjected to room temperature stress rupture tests by the progressive loading method, with the following results:

Inches removed Test No. per side by Stress (p.s.t.) Hours chemicalmilling 130,000 5.0 1 .0000 140,000 5.7 an 2 140, 000 1 10. 130,000 5.5 3 0075 140, 000 10. 2 150, 000 0.1 130, 000 5.3 4 0075 140, 000 ll. 4 150,000 0.1 130,000 5.0 5. 0150 140, 000 5. 8 150,000 0.1 130, 00 6.2 6 .0160 140, 000 11. 1 150, 000 1 0.2

Ruptured.

All of the specimens tested were as" wide, having side notches 0.125 deep with 60 included angle and 0.005" root radius, the notches not being out until after the test specimens had been chemical-milled. Prior to chemical milling, the specimens were heated and given a 1000 F. stress relief, and air cooled.

The chemical-milled pieces have a bright, clean surface which is relatively smooth, only slightly matted, and usually has a surface finish as good as R.M.S. 25.

The hardness of the alloy is not substantially affected by the chemical-milling, and a stress-relieved sheet of the alloy PWA 685 which had an initial hardness (Rockwell C scale) of RC 33 to 34.5, had a hardness of RC 34.5 to 36 after being chemical-milled to a depth of 0.0075" and a hardness of RC 34.5 to 37 after being chemical-milled to a depth of 0.015".

The chemical-milled test specimens showed a slightly superior performance in bend tests compared with the pieces of the same alloy under the same test conditions.

The mordant or etching bath of the present invention is also suitable for use in the etching of titanium metal and its alloys prior to welding. The various operations in connection with the production of the front compressor case for a jet-turbine airplane engine involving the welding of the casing may be cited as an example of the time-saving effected by etching with the present invention, and the chemical cleaning of the parts by the mordant prior to welding, saves as much as 50 hours on each compressor case.

The mordant is allowed to act on the parts, at least in those areas to be welded, to remove approximately 0.0015" to 0.002 thickness on each surface of the metal, thereby removing all of the oxide and scale which 'is normally present and is virtually unmachinable except by grinding, after which the parts may be welded together by arc welding in a helium or an argon atmosphere No hydrogen embrittlement follows the welding, nor do any cracks normally develop during or after completion of the welding operation.

When the rapid action of the mordant has ceased, the bath is preferably replaced by a new bath.

The invention in its broader aspects is not limited to the specific compositions, processes and steps shown and described but departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

1. An etching bath consisting essentially of an aqueous solution of from 4.2% to 6.8% by weight of hydrofluoric acid, from 9.27% to 12.53% of nitric acid and from 0.0015% to 0.08% of a wetting agent selected from the group consisting of sodium alkyl sulfates and sulfated fatty alcohols.

2. An etching bath consisting essentially of an aqueous solution of 5.5% by weight of hydrofluoric acid, 10.9% of nitric acid and 0.04% of a wetting agent selected from the group consisting of sodium alkyl sulfates and sulfated fatty alcohols.

3. The process of etching titanium and its alloys which comprises subjecting the titanium to the etching action of a bath consisting essentially of an aqueous solution of from 2.4% to 7.4% by weight of hydrofluoric acid, from 5.9% to 21.2% of nitric acid and from 0.0015 to 0.08% of a wetting agent selected from the group consisting of sodium alkyl sulfates and sulfated fatty alcohols.

4. The process of etching titanium and its alloys which comprises subjecting the titanium to the etching action of a bath consisting essentially of an aqueous solution of from 4.2% to 6.8% by weight of hydrofluoric acid, from 9.2% to 12.5% of nitric acid and from 0.0015% to 0.08% of a wetting agent selected from the group consisting of sodium alkyl sulfates and sulfated fatty alcohols.

5. The process of etching titanium and its alloys which comprises subjecting the titanium to the etching action of a bath consisting essentially of an aqueous solution of 5.2% by weight of hydrofluoric acid, 10.9% of nitric acid and 0.04% of a wetting agent selected from the group consisting of sodium alkyl sulfates and sulfated fatty alcohols.

References Cited in the file of this patent UNITED STATES PATENTS 2,261,700 Ryznar Nov. 4, 1941 2,640,765 Easley et a1. June 2, 1953 2,711,364 Beach June 21, 1955 --2,724,667 MacPherson Nov. 22, 1955 2,753,337 Klug July 3, 1956 2,801,909 Hirdler Aug. 6, 1957 2,828,194 Hopkins Mar. 25, 1958 2,847,286 Neunzig et al Aug. 12, 1958 2,856,275 Otto Oct. 14, 1958 2,876,144 Bomberger et a1. Mar. 3, 1959 OTHER REFERENCES Moneypenney: Stainless Iron or Steel," published by Chapman & Hall, London, vol. 1, p. 183, lines 7 and 8. Mellor: A Comprehensive Treatise on Inorganic and Theoretical Chemistry,. published by Longmans, Green & Co., New York, 1927, vol. 7, p. 19, lines 30-32.

Metal Finishing, October 1953, pp. -67. 

1. AN ETCHING BATH CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF FROM 4.2% TO 6.8% BY WEIGHT OF HYDROFLUORIC ACID, FROM 9.27% TO 12.53% OF NITRIC ACID AND FROM 0.0015% TO 0.08% OF A WETTING AGENT SELECTED FROM THE GROUP CONSISTING OF SODIUM ALKYL SULFATES AND SULFATED FATTY ALCOHOLS. 